In the automotive industry in particular, there is a continuous need to lighten vehicles and to increase safety by using and by joining light steels or steels presenting high tensile strength to compensate lower thickness. Thus, several families of steels like the ones mentioned below offering various strength levels have been proposed.
Firstly, steels have been proposed that have micro-alloying elements whose hardening is obtained simultaneously by precipitation and by refinement of the grain size. The development of such steels has been followed by those of higher strength called Advanced High Strength Steels which keep good levels of strength together with good cold formability.
For the purpose of obtaining even higher tensile strength levels, steels exhibiting TRIP (Transformation Induced Plasticity) behavior with highly advantageous combinations of properties (tensile strength/deformability) have been developed. These properties are associated with the structure of such steels, which consists of a ferritic matrix containing bainite and residual austenite. The residual austenite is stabilized by an addition of silicon or aluminum, these elements retarding the precipitation of carbides in the austenite and in the bainite. The presence of residual austenite improves ductile behavior Under the effect of a subsequent deformation, for example when stressed uni-axially, the residual austenite of a part made of TRIP steel is progressively transformed to martensite, resulting in substantial hardening and delaying the appearance of necking.
To achieve an even higher tensile strength, that is to say a level greater than 800-1000 MPa, multiphase steels having a predominantly bainitic structure have been developed. In the automotive industry or in industry in general, such steels are advantageously used for structural parts such as bumper cross-members, pillars, various reinforcements and abrasion-resistant wear parts. However, the formability of these parts requires, simultaneously, a sufficient elongation, greater than 10% and not a too high yield strength/tensile strength ratio so as to have a sufficient reserve of plasticity.
All these steel sheets present relatively good balances of resistance and ductility, but new challenges appear when it comes to assemble these sheets using for instance conventional spot welding techniques. Henceforth, new concepts presenting, high strength and high formability while being weldable using existing welding techniques are needed.
So as to reduce body in white weight, European application EP1987904 aims at providing a joint product of a steel product and an aluminum material, and a spot welding method for the joint product, ensuring that spot welding with high bonding strength can be performed. In one embodiment, a steel product having a sheet thickness ti of 0.3 to 3.0 mm and an aluminum material having a sheet thickness t2 of 0.5 to 4.0 mm are joined together by spot welding to form a joint product of a steel product and an aluminum product. In this joint product, the nugget area in the joint part is from 20×t20.5 to 100×t20.5 mm2, the area of a portion where the thickness of the interface reaction layer is from 0.5 to 3 μm is 10×t20.5 mm2 or more, and the difference between the interface, reaction layer thickness at the joint part center and the interface reaction layer thickness at a point distant from the joint part center by a distance of one-fourth of the joint diameter Dc is 5 μm or less. According to this construction, there is provided a dissimilar material joint product with excellent bonding strength, which can be formed by an existing spot welding apparatus at a low cost without using other materials such as clad material. This is done without adding a separate step and a spot welding method for the dissimilar material joint product. Such a method implies welding steel sheet to an aluminum one, the joint material resistance will have a soft area on the aluminum side compared to the steel one.
US application US2012141829 comes up with a spot welded joint which includes at least one thin steel plate with a tensile strength of 750 MPa to 1850 MPa and a carbon equivalent Ceq of equal to or more than 0.22 mass % to 0.55 mass % and in which a nugget is formed in an interface of the thin steel plates. In the nugget outer layer zone, a microstructure consists of a dendrite structure in which an average value of arm intervals is equal to or less than 12 μm, an average grain diameter of carbides contained in the microstructure is 5 nm to 100 nm, and a number density of carbides is equal to or more than 2×106/mm2. Such application does not aim at third generation steels but conventional ones only.